1. Field of the Invention
The present invention relates to a method of establishing halftone dot thresholds and an apparatus for generating binary data, suitable for use in an image recording apparatus or the like for producing a halftone dot image film, and an image film/plate-making system including such an image recording apparatus for producing a halftone dot image film or plate.
2. Description of the Related Art
There has recently been known a halftone dot generating technique for establishing a supercell on a pixel grid determined by an output resolution, dividing the supercell into halftone dot cells, and assigning a threshold to each of pixels in the halftone dot cells for thereby establishing a halftone dot threshold.
For details of a process of generating halftone dots in relation to a supercell, see, for example, "POSTSCRIPT SCREENING" written by Peter Fink, published by Adobe Systems Incorporated in 1992.
By making up a supercell of a plurality of halftone dot cells, it is possible to change the screen ruling and dot angle in smaller intervals for thereby selecting values closer to indicated screen ruling and dot angle settings.
A pixel grid comprises a set of pixels as blackening units. The pixel grid may be imaged as a matrix of vertical and horizontal arrays of pixels.
As shown in FIG. 12A of the accompanying drawings, a halftone dot cell H comprises 10 pixels P (actually, more pixels, e.g., 256 pixels, depending on the gradation of an image to be produced, on a pixel grid. The halftone dot cell H is usually represented by a square shape as indicated by the dot-and-dash line.
In FIG. 12A, the halftone dot cell H has a dot angle of (1/3) rational tangent RT {RT=(1/3)}!.
FIG. 13A of the accompanying drawings shows a halftone dot cell H having a dot angle of 0.degree. rational tangent RT (RT=0)!.
Thresholds (not shown) that are assigned to the respective pixels of the halftone dot cell H are of values progressively greater outwardly from the center of the halftone dot cell H. Therefore, the pixel P at the center of the halftone dot cell H is first in the blackening sequence. In FIG. 13A, the pixel P which is in the first place in the blackening sequence is blackened as shown hatched.
The halftone dot cell H which is blackened successively from its center is represented by a square shape with a hatched circle placed therein, as shown in FIG. 13B of the accompanying drawings which corresponds to FIG. 13A. The halftone dot cell H shown in FIG. 12A, which is blackened successively from its center, is also represented by a square shape with a hatched circle placed therein, as shown in FIG. 12B of the accompanying drawings which corresponds to FIG. 12A. In FIG. 12B, the halftone dot cell H is tilted the same angle as the halftone dot cell H shown in FIG. 12A.
FIG. 14 of the accompanying drawings schematically shows a supercell S which is made up of nine halftone dot cells H1.about.H9. As shown in FIG. 14, the supercell S has four vertexes 2.about.5 which are required to be aligned with vertexes of respective pixels P. Vertexes 6, 7, 8, for example, shared by the halftone dot cells H1.about.H9 do not need to be aligned with vertexes of pixels P.
As shown in FIG. 14, the supercell S is placed in a two-dimensional plane having perpendicular x- and y-axes. If the number of pixels on the x-axis from the origin to the vertex 2 is "m" and the number of pixels on the y-axis from the origin to the vertex 3 is "n", then the supercell S has a rational tangent RT=(n/m) which corresponds to dot angle.
In each of the halftone dot cells H1.about.H9, there are established thresholds of 0, 1, 2, . . . , 255 assigned to respective pixels that are positioned in a spiral pattern extending outwardly nearly from the center of the halftone dot cell.
A halftone dot cell in which thresholds are assigned respectively to pixels is referred to as a "halftone dot cell threshold template". A supercell in which thresholds are assigned respectively to pixels is referred to as a "supercell threshold template".
For the sake of brevity, it is assumed that the size of pixels of an original image to which such a supercell threshold template is applied is the same as the size of the halftone dot cell H, so that a blackening process according to a so-called density pattern method will be considered.
If the image data of such an original image are of a value close to 255 where the halftone dot percentage is 100%, then each of the halftone dot cells H1.about.H9 is in an almost fully blackened state.
Each of the halftone dot cells H1.about.H9 in which the thresholds are arranged as described above is blackened successively nearly from its center, with a highlight point (having a value of 0 in the above example) positioned near its center. Such a halftone dot cell is referred to as a halftone dot cell with a highlight point at center, or a halftone dot cell with a small point at center because its blackened point is generally small on the highlight size. The halftone dot cells H shown in FIGS. 12B and 13B are a halftone dot cell with a highlight point at center.
In the supercell S composed of the halftone dot cells H1.about.H9 each with a highlight point at center, however, the number of pixels which are not periodically blacked, i.e., the number of blank pixels, varies in the vicinity of the vertexes 2.about.8 of the halftone dot cells H1.about.H9. Specifically in an extreme example, the vertex 6 is blackened, the vertex 7 is left blank, and the vertex 8 is blackened.
Within the supercell S, blackened pixels in the halftone dot cells H1.about.H9 are distributed substantially uniformly. However, the number of blank pixels cannot be controlled, and periodically varies in the halftone dot cells H1.about.H9. Such a periodic variation in the number of blank pixels results in a grid-like or striped moire, which may be considered to be a moire caused by the frequency of the halftone dots and the output resolution.
FIG. 15 of the accompanying drawings shows a specific simulated image of moire stripes produced on an output image 9 when image data are outputted with a high halftone dot percentage to generate a shadow area. It can be seen from FIG. 15 that the moire stripes appear clearly in the direction indicated by the arrows 10. In FIG. 15, square dots 11 represent blank pixels, with other pixels being blackened.